Embodiments disclosed herein relate to an air float bar for use in positioning, drying or curing a continuous generally planar flexible material such as a web, printed web, newsprint, film material, or plastic sheet. More particularly, they pertain to an air float bar whose pressure pad area includes an infrared light source, such as an infrared bulb, a reflector surface and a lens to enhance accelerated infrared heating of web material to cause solvent evaporation, drying and/or curing. Electromagnetic infrared heat energy in combination with jets of air impinging upon the web surface provide for concentrated heating of the web material, thereby providing subsequent rapid evaporation, drying and/or curing from the surface of the material.
U.S. Pat. No. 5,035,066 (Wimberger) teaches the integration of an infrared emitter into a Coanda-type flotation air bar. Cooling air is brought through a channel assembly that encloses the emitter. A quartz lens is used to enclose the emitter while allowing transmission of electromagnetic energy in the range of infrared wavelengths to pass from the channel assembly enclosure to the web. In one embodiment, said cooling air, after passing around the emitter inside said channel assembly, is discharged through holes in a quartz lens of said emitter channel assembly. Although this arrangement provides some recovery of heat by discharging said cooling air to the web surface after flowing around said emitter, the flow path is not optimized for both cooling of the emitter and recovering of heat to the air which is subsequently impinged on the web. The prior art arrangement with passage of air through holes in the quartz lens does not provide optimum fluid contact to effectively cool the emitter and lens as is desired in order to maintain longevity of these components against thermal degradation or contamination. Nor does it maximize the recovery of heat from the emitter, lens and reflector. It is further desirable to keep the emitter and lens free from contamination by aggressive solvent vapors, liquids such as inks and/or coating materials, and other contaminants such as paper dust or chards of material from broken webs. Cooling and prevention of contamination of the reflector is also desirable for the same reasons as discussed for the lens. If such contamination occurs, the infrared energy is absorbed by the quartz material of the emitter and quartz lens instead of being transmitted through said quartz to the web surface, which results in loss of drying and heat transfer efficiency, and also promotes thermal degradation as the design temperatures of the emitter and lens materials may easily be exceeded. Similarly, contamination will reduce the reflectivity of the reflector resulting in loss of drying and heat transfer efficiency and material thermal degradation.
As is known to those skilled in the art of infrared dryers, it is desired to prevent possible ignition of combustible materials, such a paper web, should said combustible materials come into contact with hot surfaces. It is further desired to have a quick acting means of interrupting the heat flux from the infrared emitter from reaching the web to prevent ignition of a stationary or broken web. A means of blocking the infrared heat flux is taught in U.S. Pat. Nos. 6,049,995 and 6,195,909 (Rogne et al.) but requires detection and an active mechanical means to assure that the web is not exposed to temperatures exceeding the ignition temperature of materials being processed. As is known to those skilled in the art, it is often desirable to use fast-cooling tungsten or carbon filament emitters as are available from Heraeus Noblelight of Hanau, Germany. These fast-cooling elements minimize the time necessary to bring the infrared heat flux and associated surface temperatures low enough avoid ignition of said combustible materials should the web stop or break during an upset to the drying process. Even with such quick cooling emitters, it is desirable to keep the exposed surfaces of the air float bar as cool as possible at all times to prevent possible ignition of said combustible materials, even when web stoppage or a web breakage upset may go undetected.
It is also known to those skilled in the art of drying materials by means of infrared energy that the amount of heat effectively absorbed by the material is dependant on a number of key factors, including the temperature of the emitter, the geometry defining the infrared light paths to the materials, and the absorption characteristic of the materials to be dried. It is desired to select an emitter type such that its temperature will emit maximum electromagnetic energy flux in the range of wavelengths that correspond with the wavelengths of maximum absorption in the material to be dried. In the case of a coated web the materials typically include the base web substrate, and a coating comprised of solids, and a solvent such as water or an organic solvent, said solvent to be dried. Each of these materials exhibits an infrared absorption characteristic as a function of infrared wavelength, or spectra, which is to be considered in the selection of the type of emitter to be used.
In some cases, such as printing, the coating or ink is not applied to the substrate uniformly in all areas. It such cases it is desirable to maximize the infrared energy flux to the areas having coating or ink while minimizing the energy flow to uncoated (unprinted) areas. The locations of the coated and uncoated areas are variable according to the product to be dried. One prior art method used to effect the direction of drying energy to areas requiring drying while limiting energy to areas not requiring drying prescribes the selection of the emitter such that it will provide high infrared heat flux at a range of wavelengths that match high absorption wavelengths for the solvent, while minimizing the emission of infrared energy at wavelengths where absorption in the dry solids and the substrate is low. Another prior art method arranges a plurality of emitter lamps in an array wherein the emitter lamps may be activated (energized) or deactivated (de-energized) to emit infrared energy approximately matching the physical location of the areas to be dried. In the drying of moving continuous webs having widely variable patterns of printed and unprinted areas, this method of activating and deactivating a fixed array is only practically capable of directing drying energy on a spatially coarse scale. The infrared energy can be applied more or less in lanes along the length of the web to be dried, which does not address the need to limit drying heat to the unprinted areas that lie between printed areas along the direction of web travel.